It is common knowledge that the illuminative output of a semiconductor laser diode is not linear across its entire operating range. More specifically, in a typical graph of light output as a function of a voltage differential across the diode, the resulting curve has a lower level, nonlinear operating region, joined by a knee region to a higher level operating region where light output varies linearly as a function of the voltage differential.
The laser diode has been used frequently as a light source in a variety of photographic applications. For example, in the field of laser printing, a laser beam is focused through a lens and then scanned onto a negative film. The diode is often controlled by digital image data stored in a computer. In some systems, the laser is operated so that it is completely on or off, thereby generating either black or white pixels to form an image in response to the image signals. However, image quality may be improved by forming the image from pixels having varying gray tones. More specifically, when gradations in intensities of the gray levels is such that the transitions between pixels appear relatively smooth, this is referred to as a "continuous tone" image.
In order to generate these varying gray tones, the laser diode can be conventionally modulated so as to adjust its light output over a limited range. However, creating the desired gray tones from digital image data across a spectrum of white to black, can present a problem when the diode does not operate in a linear manner across its operating range from essentially zero light output to maximum illuminative intensity.
Conventionally, a number of laser diode control systems have been disclosed. For example, in U.S. Pat. No. 4,009,385 by Sell, there is disclosed a circuit for prebiasing a laser diode with a current near the diode's threshold point so as to minimize any delay in the laser responding to an input signal.
Another controller for a laser diode, described in U.S. Pat. No. 4,592,057 by Comerford, utilizes a programmable current source which is responsive to an error signal in order to drive the laser.
In U.S. Pat. No. 4,339,822 by Kolodzey there is disclosed a method for modulating a laser diode wherein a TTL or ECL signal input is converted to an ECL compatible output which is then amplified sufficiently to drive the diode.
Another prebiasing circuit is disclosed in U.S. Pat. No. 4,385,387 by Trimmel, in which a laser diode is prebiased for operation above the knee of a light-current characteristic curve.
Chapman, in U.S. Pat. No. 4,412,331, discloses a circuit for controlling the DC bias current of a laser diode so as to operate the diode at its knee.
In addition, in U.S. Pat. No. 4,583,128 by Anderson, Jr. et al there is disclosed a system for providing continuous tone images in which a photodiode continuously monitors the optical output of a laser diode and provides a feedback signal for adjusting the laser output.
Furthermore, Meuleman et al in U.S. Pat. No. 4,450,564, discloses a feedback circuit for reducing nonlinear conversion characteristics and other variations in a laser diode by utilizing a photodiode which senses the output of the laser diode and generates a negative feedback signal to adjust the bias current of the laser diode.
And finally Holland, in U.S. Pat. No. 4,237,427, discloses a laser diode whose operation is affected by a first control loop which monitors the intensity of the laser output to develop a current signal for biasing the diode in response to a signal having a balanced duty factor, with a second control circuit operating to shut off the diode if the input signal duty factor becomes unbalanced.